This invention relates to exposure adjusting devices, and more particularly to an exposure adjusting devive for a microfilm copying machine.
In general, a microfilm copying machine copies an enlarged image of a microfilm. There are two types of microfilm copying machines: one in which the image of the microfilm is copied at a predetermined magnification only, and another in which the image of the microfilm may be copied at different magnifications. In the former copying machine, the quantity of light output by the exposure lamp may be constant; however, in the latter copying machine the quantity of light must be adjusted according to the selected magnification, so as to obtain a correct exposure value.
Furthermore, in a conventional microfilm copying machine, there are variations in the exposure of a photo-sensitive body which are caused by a change in the light quantity due to the soiled condition inside the copying machine, variations of the intensity or quantity of transmitted light due to a change of the film density, etc. These variations are troublesome in the conventional copying machine.
In a conventional microfilm copying machine, the quantity of light is controlled by merely manually adjusting the quantity of light emitted by the exposure lamp. Accordingly, if the variation range of the enlargement magnification is large, the quantity of light must be adjusted over a wide range, and accordingly, the wavelength of the light emitted by the exposure lamp is also greatly changed. As a result, light of wavelengths deviating from the inherent sensitivity range of the body or drum is received by the body or drum, adversely effecting the copying operation.
Furthermore, the conventional microfilm copying machine has no means for automatically correcting the variation of the exposure due to soiled conditions inside the copying machine such as for instance a soiled lens system, and requires manual adjustment when the intensity of the light source is changed due to the replacement of the exposure lamp.